“Old” and “new” natural-law theorists occasionally come to different conclusions. (For instance, traditional natural-law theory holds that capital punishment is permissible in principle even if not always in practice, while Grisez, Finnis, and George maintain that it is always and in principle wrong.) But the differences in method are profound even where the theories agree in their conclusions, as they do on matters of sexual morality. In my view, the weakest parts of Conscience and Its Enemies — indeed, the parts least likely to convince readers who are neutral in the debate over “same-sex marriage” — are those in which George deploys “new natural law” arguments concerning sexuality.
For the “old” or traditional natural-law theorist, what is good for human beings, as for other living things, is defined in terms of the ends or goals they must realize in order to flourish as the kinds of creatures they are. A squirrel needs to gather seeds, nuts, and the like if it is to feed itself. This end or goal (what followers of Aristotle would call a “final cause”) is partially definitive of what is good for squirrels, and remains so even if the occasional squirrel has for whatever reason (genetic defect, say) no desire to gather seeds or nuts. Human beings too need to realize the ends or final causes of their natural capacities if they are to flourish, and this includes the ends of their sexual capacities. These ends are both procreative and unitive; that is to say, our sexual faculties are by nature “aimed” at getting us to mate, and to bond emotionally, with a person of the opposite sex. That some people’s sexual organs are damaged or worn out and some people’s sexual desires are distorted in various ways doesn’t change their natural end, any more than a squirrel that is missing a leg due to an accident or birth defect fails to be the sort of creature that by nature is four-legged.